The study is designed to determine the role of fungal hypersensitivity in the prognosis of cystic fibrosis (C.F.). Prior studies have demonstrated serum precipitins to Aspergillus and Candida antigens in 50% of C.F. patients, and there was highly significant correlation of these antibodies with clinical severity and death. Those who fared better tended to have less precipitin activity and more IgE responses. It is proposed to follow at least 40 C.F. patients a minimum of 20 months. During this time, clinical evaluation including the Shwachman score will be compared with immunological parameters for Type I (IgE mediated) and Type III (immune complex) reactions. If immune injury can be found to play an important part in C.F. pulmonary disease, therapy aimed at resolution of this process should improve survival and morbidity.